The present invention relates generally to spacecraft systems, and more particularly to solar array-mounted stationkeeping and maneuvering thrusters.
Thrusters are conventionally mounted on a surface of a central inboard body portion of a spacecraft, such as a satellite, and are typically canted about 45 degrees or more such that as the solar arrays on the spacecraft rotate, the exhaust streams of the thrusters do not impinge on the solar arrays. The exhaust streams of thrusters having high specific impulses (Isp) in the range of about 1600 to 2500 seconds, such as Hall Effect thrusters, electrical arc (Arc-jet) thrusters and ion engines, are capable of eroding exposed spacecraft surfaces in their path because of the high velocity of the emitted gases. Thrusters which fall into the high Isp category typically employ electrical power to enhance the energy of the exhaust stream. The erosive process of these exhaust streams can then degrade the electrical, thermal and/or optical properties of the exposed surfaces, such as the solar arrays, leading to a loss in spacecraft system performance. An example of this effect would be impingement of the exhaust streams of north-south stationkeeping thrusters on deployed solar array surfaces, a process which may lead to premature degradation of array power output capability.
To mitigate the effects of surface erosion, thrusters are generally canted by about 45 degrees or more to reduce the magnitude of the thruster plume impingement on sensitive spacecraft surfaces. Canting of the thrusters results in a decrease in the effective Isp, with the decrease being generally proportional to the cosine of the cant angle. Consequently, the spacecraft must carry larger amounts of propellant for stationkeeping and/or maneuvering. Generally, about 30 to 40 percent more propellant must be carried to compensate for the thrust which is typically lost due to canting. The weight availability for spacecraft payloads is decreased as a result. The loss in effective thruster performance adversely affects various factors, including mission life, payload revenue production and/or performance capability and launch vehicle costs.
What is needed therefore is an apparatus for directing thruster exhaust away from the spacecraft, which, in addition to reducing the erosive effects of thruster exhaust, delivers an increase in the effective Isp of the thruster.